Radio receiving system



P. o. FARNHAM RADIO RECEIYING SYSTEM July 27, 1937.

Original Filed April 11, 1934 3mm: gyflw,

Patented July 27, 1937 UNITED STATES RADIO RECEIVING SYSTEM Paul O. Farnham, Mountain Lakes, N. J assignor, by mesne assignments, to Radio Corporation of America, New York, N. Y., a corporation of Deiaware Application April 11, 1934, Serial No. 720,113

Renewed .iunc 6, 1936 12 Claims This invention relates to radio receiving systems and particularly to radio receivers of the automatic gain control type.

A This-application is a continuation-impart of my c opending application Ser. No. 641,280, filed Nov. l, 1932, Patent No. 2,003,100, May 28, 1935.

Various circuit arrangements have been proposed for automatically regulating the sensitivity of a radio receiver to maintain a radio voltage level at the detector which is approximately con stant over a wide range in the magnitude of the radio voltage at the receiver input terminals. The advantage of such receivers is that the operator need make little or no adjustments to obtain a constant audio output level when tuning to signals of different intensity or when tuned to a signal which tends to vary in intensity. The general method previously employed to apply an automatic gain control to the radio amplifier portion of a receiver has been to rectify the carrier component of an amplified radio voltage to produce a direct current voltage that varied as a substantially linear function ofthe radio input to thedetector, and to impress suchdirect current voltage'upon the radio amplifier as a gain control voltage. The application of the prior types of automatic gain control systems to radio amplifiers of-high selectivity has resulted in an apparent, ora real, loss in selectivity as the audio output did not fall off until the receiver was detun edfar from resonance at the frequency of a-nearby broadcasting station. Unfortunately, however, the quality of reproduction fell off rapidly with detuning, although the output level remained constant, and tuning or resonance meterswere required in addition to the usual tuning scale.

The, known types of automatic gain control receivers are well exemplified by' the patent to Graham, No. 1,851,387, which illustrates the automatic gain control system employed in the Model No. 846 radio receiver manufactured commercially by the Stromberg-Carlson Telephone Manufacturing Company. This particular receiver included a tuning meter and, as stated in the Engineering Data Book for Stromberg-Carlson No. 846 Radio Receiver, page 9:

The meter must be used for this indication as, in the case of powerful signals, the volume does not change for a certain distance each side of the resonance point.

vide a radio receiver of the automatic gain control type which is free from the objections of the prior receivers of this type. An object is to provide a radio receiver of the automatic gain control type which is characterized by high selectivity. A further object is to provided methods of and apparatus for automatic gain control in radio receivers which are characterized by a greater selectivity in the transmission of amplified radio-frequency voltages impressed on the detector element of the receiver than in the transmission of the amplified radio voltage to the rectifier of the automatic gain control system.

These and other objects of the invention will be apparent from the following specification when taken with the accompanying drawing in which:

Fig. l. is a fragmentary circuit diagram of a superheterodyne radio receiver embodying the invention; and

Fig. 2 is a fragmentary circuit diagram of a .tuned radio frequency amplifier embodying the invention.

In the drawing, the reference character I identifies a radio frequency amplifier, including one or more vacuum tubes, a local oscillator and a frequency changer, which receives carrier Wave energy from an appropriate collector structure and which works into a detector and audio amplifier or other load circuit. The exact design and construction of the superheterodyne system is not an essential part of the present invention, and only the last tube 2 of the intermediate frequency amplifier is illustrated.

The tuned output circuit 3 of the last amplifier stage is coupled, with critical or slightly greater than critical coupling, to the tuned input circuit d of the detector and automatic contrclstage of the receiver, the radio voltage E which is built up across the primary circuit 3 establishing a radio voltage E across the secondary circuit i. The single tube 5 of the combination detector-control stage may be a tube known as type 55 and includes a pair of rectifying anodes A1, A2, a control grid G, plate P and a cathode K.

Referring first to the circuit elements which cooperate with the tube 5 to give the stage the function of a detector and an audio frequency amplifier, the high potential terminal of the tuned input circuit d is directly connected to the anode A2, and the opposite terminal is connected to the cathode K through an audio frequency resistance ti and an audio frequency by-pass con-- denser i, the resistance ti being lay-passed for radio, frequency by a condenser ii. The audiofrequency voltage developed by diode rectification across the resistance 6 is impressed upon the control grid. G througha filter comprising re sistance 9 and a radio frequency condenser Hi. The plate circuit includes the primary winding of an audio frequency transformer ll, preferably provided with means such as a shunt resistance l2 and adjustable tap lit for controlling the output level of the receiver system, and a source of direct current potential +B which is connected to ground or -B through the voltage divider formed by resistances M, l5. The cathode K is connected to ground through a resistor It which is by-passed for audio frequency by the condenser H. A direct current path for impressing a bias voltage between the anode A2 and the cathode K is completed by the resistance I8 which is connected between the resistance 6 and the junction of the resistances I4, E5, of the voltage divider.

The flow of plate current through the cathode resistor l 6 will establish a potential drop E1 which places the cathode K at a more positive direct current potential than that of ground or B. The direct current voltage E2 across the voltage divider M, i5 gives rise to a potential E3 across resistance M which applies a positive potential to the anode A2.

The bias voltage on the anode A2 is equal to the difference of the voltages E1 and E3. The voltage E3 and the value of resistance l6 are so chosen that, when the input voltage E is zero, a suificient plate current will flow in the tube 5 to make the voltage E1 greater than the fixed voltage E3. In other words, the plate current has a normal value in the absence of received signals which is suflicient to maintain the anode A2 and the control grid G at direct current potentials which are negative with respect to the cathode. It therefore follows that small peak values of E less than E1-Es will not be rectified in the anode Aa-cathode circuit and hence no audio frequency voltage will be developed across the resistor 6.

Attention is directed to the fact that the connection between the audio frequency resistance 5 and the grid G will impress both the rectified audio voltage and the rectified direct current voltage upon the grid G, thus causing the tube 5 to function. as an amplifier for any direct current voltage developed across resistance 6. This amplified direct current voltage operates to reduce or remove the bias voltages normally impressed upon the anode A2 and the grid G. When the peak value of the radio input voltage E exceeds the bias voltage E1E3 on the anode A2, rectification takes place in the circuit including the elements m, G and an additional negative bias is thus impressed upon the control grid, which bias effects a reduction in the plate current flow. The consequent reduction in the voltage E1 across resistance it removes the initial bias E1E3 on the anode A2, and the normal diode detection then takes place in that circuit. The direct current voltage developed across resistance 6 by rectification, together with the decrease in the voltage E1 across the cathode resistance 55 serves to maintain the control grid bias at a value suitable for audio frequency amplification.

A bias voltage for automatically controlling the gain of amplifier l is developed between anode A1 and cathode K by connecting anode A1 to the tuned primary circuit 3 through a radio frequency condenser is and to ground through a resistance 2B. The anode terminal of resistance 2i] is connected through a filter comprising resistance 2! and condenser 22 to the lead 23 which extends to the control grid circuits of the tube or tubes of amplifier I.

It is desirable that the magnitudes of the radio voltages, E and E0, be so'related that when the detector input E is increased to that value sulficie-nt to make the voltage drop E1 across the cathode resistance iii equal to the fixed bias component E3, the peak voltage E0 on the automatic gain control anode will be equal to E1, and thus initiate the action of the automatic gain control. To make the voltage E0 greater than E, the inductance of the primary tuned circuit 3 may be larger than that of the secondary circuit 4.

The operation of the circuit is as follows. So long as the peak detector input voltage E is less than the negative bias voltage, E1E3, on the anode A2, there will be no rectification of the impressed voltage E by the diode A2K. Nor will there be any incidental rectification of radio voltages in the triode amplifier circuit since the filter, resistance 9 and condenser I0, prevents radio voltages from reaching grid G.

The voltage E1 due to plate current fiow with no input to the amplifier elements is relatively high and transmission of any audio frequency signals by the amplifier is effectively suppressed. When the peak voltage E exceeds the negative bias on anode A2, rectification takes place and the direct current and audio frequency voltages developed across resistance 6 are impressed on the grid G. The rectified direct current voltage increases the negative bias on grid G, thus reducing the plate current and thereby reducing the voltage E1. This reduces the delay action bias voltage on anode A2, and reduces the negative bias on grid G to a value appropriate for audio amplification.

Turning now to the automatic gain control, the delay action bias voltage on anode A1 is equal to E1. As soon as rectification takes place in the detector diode circuit, the voltage E1 is reduced by the direct current amplifier action of the triode elements and the delay bias is thus removed from, the automatic gain control diode. The rate at which'a gain control voltage is developed, with increasing radio input voltage E0, is therefore more rapid than is the case when a fixed delay bias voltage is impressed upon the gain control rectifier. This method of and. circuit arrangements for removing the delay bias impressed on a gain control rectifier are described and claimed in my copending application Ser. No. 641,279, filed Nov. 4, 1932.

The important feature of the described circuit is the feeding of the rectifier A1K from the primary tuned circuit 3 and the feeding of the detector A2K from the secondary tuned circuit 4. As is customary in the design and construction of intermediate frequency transformers, the tuned circuits are coupled with critical or slightly greater than critical coupling to obtain high selectivity in the intermediate frequency amplifier. The significance of this circuit arrangement and this degree of coupling is that the selectivity of transmission to the detector is greater than the selectivity of transmission to the rectifier of the gain control system. It is also advantageous for the purpose of obtaining diiferential selectivity effects to have the power ratio of the secondary circuit less than that of the primary circuit.

The effective result of this differential selectivity is that the radio input to the rectifier does not fall ofi as rapidly with detuning from exact resonance at a transmission channel as does the input to the detector. The audio output therefore falls off rapidly with detuning since the gain is not increased as an inverse function of the input to the detector, but as an inverse function of the input to the rectifier. The -differential selectivity of the receiver.

Substantially the same arrangement-may be usedin receivers of the tunedradio frequency type in which the detector works out 'of 'a pair of tuned circuits. coupled with 'critical orsomle- What greater than critical coupling. The' invention may be applied tothe customary type o-f cas caded radio amplifier employing only one tuned circuit in each interstage coupling, as shown in Fig. 2. A twostage radio amplifier employingscreen grid tubes 24 is illustrated but it is to be understood that the exact number of cascaded stages is not material. The tuned input circuit of the first amplifier tube is connected in any appropriate manner with the antenna A and similar tuned input circuits 26 are provided for the subsequent radio amplifier tube or tubes 24 and.

for the detector tube 21, the tuning condensers of all of the circuits being preferably mechanically connected, as indicated. by the broken line 28, for simultaneous adjustment. The inductances of the tuned circuits 26 are closely coupled to the plate inductances 29 of the screen grid amplifier tubes 24 of the preceding stage, as is customary l the second amplifier tube 24 is connected to anin the design of coupling circuits-for use with screen grid amplifiers. Anappropriate method for obtaining the close coupling between the untuned primary and the tuned secondary isto wind the secondary as a close solenoidal-winding on a cylindrical form and to wind the primary winding directly over the secondary coil. This will provide the tight coupling which is necessary to obtain. adequate gain in a tuned radio amplifier of the illustrated type.

The grid of. the rectifier tube 30 first radio amplifier tube 24 is 'connectedto theplate of the tube 30, While the control grid of intermediate point in the resistance 32 which is connected between the plate of tube 30 and the ground or neutral point of the receiver. The

control grids of the amplifier tubes 24 are separated from their respective input circuits by blo k n n e t aedthe' w' etentia t l? minals of their input circuits are directly eorr- 1 nected to'thegroun d or neutral point.- v An inter- I di P in O h vo a d v d r 9?:531 59': prop-riate power sup-ply system isconnected to the e nd 0 u al 9 112 9 det m ne t h F current potential; of that point g the' cathode; of; the o ol. eb ine vqe sstedme Pa nt he vQ as v d -3W 1 s sgeti s r I spect to the direct current voltagqof the-plate:

nn t o The co tr f ids?? t e 1 connected through. a id: r ist n e 36 of more negative potential on thevoltageydivider" 35 to prov d a r d ha or-V the wqont ol "tube which is app p ate iorw ifi i A-.- n-, denser .31," is shunted across" the plate resistance; 32 of the controltubetoprevent thedevelopment v -;of radio ,andaudio-voltages; in, the output circuitrof the control tuber q 'Itwill be apparent that the selectivity oftra missionof amplifiedradio voltagesyto the detect.

is greater than the- =selectiY ty1of transmission; to:-

ythell l :tube.;, :The; receiver wil wthere orer;

of the auto-1 matic gain control system is, connected to thetuned from resonance at a signal frequency but the gain is not automatically increased at an in- I verse rate. I

The foregoing'examples of receiver designs for obtaining a higher selectivity at the detector than at the rectifier of the automatic gain control system are merely illustrative of the invention, and the preferred or appropriate circuit arrangement for obtaining differential selectivity with other types of amplifiers will be apparent to those skilled in the design and construction of radio receivers and amplifiers.

I claim:

1. In a radio receiver, the combination with a tuned radio amplifier working into a gain control rectifier system and into a detector, and means energized by said rectifier system for automatically controlling the gain of said amplifier, of means for imparting greater selectivity to the radio voltage impressed by said amplifier upon said detector than to the radio voltage impressed by said amplifier upon said rectifier, said secondmeans including at least two coupled circuits connected incascade between said amplifier and detector, the radio voltage for said rectifier being radio output of said amplifier, the second tuned circuit being connected to the detector, the first of said tuned radio circuits being connected to said rectifier for obtaining greater selectivity in the;

radio input to said detector than in. the radio input to said rectifier, and'the power ratio of the second of the tuned circuits being less than the power ratio of the first of the tuned circuits.

3. In a radio receiver, a tuned radio amplifier,

a detector, coupling means between said amplifier and detector including a tuned output circuit for ;said amplifier having. approximately it c l zc ing w a. t n d p i the detector, and means for automatically con? trolling the gain of said amplifier as a function oi radio input tothe. receiver,;said auto matiee in c nt m a i cl n l having an input circuit connected across the;

tuned output circuit ojffsaid amplifier, .and the amplifier tunerToutput circuit po'wer ration. exif ce ngfthat' of the detector tuned input circuit. I In r di re ver Provided w thaf s l airipl' er having a tunedloutput circuit,*-a'diode" iet ctq 'n tvqr h efat n d n v lc fi said output and; input Y circuits being reactivelf coupled; the "constants of the coupldbircuits being proportioned to impart a power ratio tothef diodeinput circuitwhichis less than that of the amplifier" output I circuit, a diode J rectifier' net+ work having its-diode'electrodes connected across the ramplifier i output circuit," said :rectifiernetwork includingca direct current. connection to .th f the;;amD f r i qr pqnse-t si ne emn lll i anationsanq h risai u psles Wed ie we e. J

have a high selectivity since the automatic gain control bias impressed on the amplifier tubes 24 is mon cathode and having their electrodes in a common tube envelope.

5. In a radio signaling system provided with a signal transmission tube having a tuned out put circuit, a signal utilization network including an electron discharge tube having a tuned input circuit, said output and input circuits being coupled, the constants of said output circuit and input circuit being proportioned to impart a power ratio to the said input circuit which is less than that of said transmission tube output circuit, means for deriving from signals transmitted through said system a unidirectional voltage which is dependent upon the transmitted signal amplitude, said last means being connected to said transmission tube output circuit in order to derive signal energy therefrom, said last means including a direct current connection to a gain control electrode of said signal transmission tube for regulating the gain of said tube in response to signal amplitude variations.

6. In a radio receiver of the type provided with a signal amplifier whose gain is to be automatically regulated in dependence upon variations in received signal amplitude, a multiple function tube comprising a cathode, a signal input grid, and at least an output plate, an audio frequency load circuit coupled to the plate, at least two anodes disposed within said tube, a transformer having a tuned primary circuit and a tuned secondary circuit connected to the output electrodes of said amplifier, said transformer circuits being tuned to a desired signal frequency, said tuned secondary circuit being coupled to the cathode and one of said anodes and including a load impedance for developing an audio frequency voltage from detected signal currents, means for impressing the audio frequency voltage upon said input grid of said tube, means connecting the cathode and second anode of said tube across said primary tuned circuit, and means electrically connected with said primary tuned circuit for developing a direct current voltage from said signal currents which varies in magnitude with the received signal amplitude, and additional means for impressing said direct current voltage upon said amplifier as a gain control voltage.

'7. In a radio receiver of the type provided with a signal amplifier whose gain is to be automatically regulated in dependence upon variations in received signal amplitude, a multiple function tube comprising a cathode, a signal input grid, and at least an output plate, an audio frequency load circuit coupled to the plate, at least two anodes disposed with said tube, a transformer having a tuned primary circuit and a tuned secondary circuit connected to the output electrodes of said amplifier, said transformer circuits being tuned to a desired signal frequency, said tuned secondary circuit being coupled to the cathode and one of said anodes and including a load impedance for developing an audio frequency voltage from detected signal currents, means for impressing the audio frequency voltage upon said input grid of said tube, means connecting the cathode and second anode of said tube across said primary tuned circuit, means electrically connected with said primary tuned circuit for developing a direct current voltage from said signal currents which varies in magnitude with the received signal amplitude, additional means for impressing said direct current voltage upon said amplifier as a gain control voltage, and said anodes being disposed outside the electron stream flowing from said cathode to said plate.

8. In a radio receiver of the type provided with a signal amplifier whose gain is to be automatically regulated in dependence upon variations in received signal amplitude, a multiple function tubecomprising a cathode, a signal input grid, and at least an output plate, an audio frequency load circuit coupled to the plate, at least two anodes disposed within said tube, a transformer having a tuned primary circuit and a tuned secondary circuit connected to the output electrodes of said amplifier, said transformer circuits being tuned to a desired signal frequency, said tuned secondary circuit being coupled to the cathode and one of said anodes and including a load impedance for developing an audio frequency voltage from detected signal currents, means for impressing the audio frequency voltage upon said input grid of said tube, means connecting the cathode and second anode of said tube across said primary tuned circuit, means electrically connected with said primary tuned circuit for developing a direct current voltage from said signal currents which varies in magnitude with the received signal amplitude, additional means for impressing said direct current voltage upon said amplifier as a gain control voltage, and a resistor disposed in the space current path flowing from said plate to said cathode for developing a direct current voltage, and means for impressing said last named voltage upon said input grid to provide a normal operating bias therefor.

9. In combination with a tube of the type including an amplifier section and a pair of independent diode sections, all of said sections having a common cathode, a source of signal waves, a transformer having resonant primary and secondary circuits both tuned to a common wave frequency, means coupling one of the diode sections across said primary tuned circuit, and providing a wave rectifier circuit which develops a direct current voltage, means coupling the other diode section across the secondary tuned circuit and providing a wave rectifier circuit developing an audio frequency voltage, and means impressing the audio voltage upon the input electrode of said amplifier section.

10. In combination with a tube of the type including an amplifier section and a pair of independent diode sections, all of said sections having a common cathode, a source of signal waves, a transformer having resonant primary and secondary circuits both tuned to a common wave frequency, means coupling one of the diode sections across said primary tuned circuit, and providing a wave rectifier circuit which develops a direct current voltage, means coupling the other diode section across the secondary tuned circuit and providing a Wave rectifier circuit developing an audio frequency voltage, means impressing the audio voltage upon the input electrode of said amplifier section, and an audio frequency load circuit connected to the output electrode of the amplifier section.

11. In combination with a tube of the type including an amplifier section and a pair of independent diode sections, all of said sections having a common cathode, a source of signal waves, a transformer having resonant primary and sec ondary circuits both tuned to a common wave frequency, means coupling one of the diode sections across said primary tuned circuit, and providing a wave rectifier circuit which develops a direct current voltage, means coupling the other diode section across the secondary tuned circuit and providing a wave rectifier circuit developing an audio frequency voltage, means impressing the audio voltage upon the input electrode of said amplifier section, an audio frequency load circuit connected to the output electrode of the amplifier section, said wave source including a transmission tube, and a direct current connection between a gain control electrode of said transmission tube and said primary rectifier circuit.

12. In combination with a tube of the type including an amplifier section and a pair of independent diode sections, all of said sections having 15 a common cathode, a source of signal waves, a

transformer having resonant primary and secondary circuits both tuned to a common wave frequency, means coupling one of the diode sections across said primary tuned circuit, and providing a wave rectifier circuit which develops a direct current voltage, means coupling the other diode section across the secondary tuned circuit and providing a wave rectifier circuit developing an audio frequency voltage, means impressing the audio voltage upon the input electrode of said amplifier section, and means in the space current path of said amplifier section for developing a delay bias for the anode of said first diode section.

PAUL O. FARNHAM. 

